Sharp R-990C Service Manual page 17

PROCEDURE
LETTER
Parts Name
Temp. fuse 150˚C
Thermal cut-out 150˚C
Thermal cut-out 95˚C
If incorrect readings are obtained, replace the temp. fuse or thermal cut-out.;
An open circuit temperature fuse 150˚C (MG) indicates that the magnetron has overheated, this may
be due to resistricted ventilation, cooling fan failure or a fault condition within the magnetron or HV
circuit.
An open circuit thermal cut-out 150˚C (CONV.) indicates that the convection motor has over heated,
this may be due to convection motor locked.
An open circuit thermal cut-out 95˚C (FM) indicates that the fan motor winding has overheated, this
may be due to blocked ventilation or locked cooling fan.
H CONVECTION HEATER TEST
CARRY OUT 3D CHECKS.
Before carring out the following tests make sure the heater is fully cool.
1. Resistance of heater
Disconnect the wire leads to the heater to be tested. Using ohmmeter with low resistance range.
Check the resistance across the terminals of the heater.
The resistance of heater is approximately 37.5 ohms.
2. Insulation resistance
Disconnect the wire leads to the heater to be tested. Check the insulation resistance between the
element and cavity using a 500V - 100Mohms insulation tester. The insulation resistance should
be more than 10Mohms in the cold start.
If the results of above test 1 and/or 2 are out of above specifications, the heater is probably faulty
and should be replaced.
CARRY OUT 4R CHECKS.
I THERMISTOR TEST
CARRY OUT 3D CHECKS.
Disconnect connector-E from the CPU unit. Measure the resistance of the thermistor with an
ohmmeter. Connect the ohmmeter leads to Pin No's E-3 and E-4 of the thermistor harness.
If the meter does not indicate above resistance, replace the thermistor.
CARRY OUT 4R CHECKS.
J MOTOR WINDING TEST
CARRY OUT 3D CHECKS.
Disconnect the leads from the motor.
Using an ohmmeter, check the resistance between the two terminals as described in the table below.
TEST PROCEDURES
Table: Temperature Fuse and Thermal cut-out Test
Temperature of "ON" condition
(closed circuit) (˚C)
This is not resetable type.
Below 130˚C
Below 75˚C
Room Temp. .................. 20˚C - 30˚C
Resistance ..................... Approx. 350k
Fan motor
Turntable motor
Convection fan motor
Damper motor
15
COMPONENT TEST
Temperature of "OFF" condition Indication ofohmmeter
(open circuit) (˚C)
Above 150 ˚C
Above 150˚C
Above 95˚C
- 155k
Table: Resistance of Motor
Motors
Approximately 295
Approximately 19.2 k
Approximately 210
Approximately 11 k
(When room temperature
is approx. 20˚C)
Closed circuit
Closed circuit.
Closed circuit.
Resistance
R-990C(S)